I hope it doesn't require too many constructs in the language that encourage premature optimization
I try not to worry about problems until they become evident. Actually, it is a small problem in OCaml (eg. favouring arrays over lists for speed.)
In Rust's case you'll write allocations conservatively anyway. Being over-conservative is certainly not encouraged any more than using stack allocations exclusively is in C. However, I think I read something by one of the devs saying that allocations in the the core library are effectively considered bugs. The idea is you still have a library to depend on when you eschew the runtime. Say what you will, Rust has an impressive amount of forethought put into it and they're not afraid to experiment.
and this should be possible for any high level language, including ocaml
Not without changes to the memory model, in OCaml and probably most others too. Sure that's possible, but it in no way resembles the state of things. Not yet for Rust either, but they talk about it like they have every intention of doing it. I don't know if they intend to do it automatically, and I want to have a compiler flag to enforce it.